Optochemical sensors find application for the determination of a wide diversity of analytes in the laboratory as well as in process systems. For example in a process, optochemical sensors based on the principle of fluorescence quenching can be used for the determination of gases that are dissolved in a fluid, such as for example oxygen or carbon dioxide. To perform this function, the sensor includes a sensitive element with a signal substance that is capable of interacting with the analyte. Under the principle of fluorescence quenching, the molecules of the signal substance are excited by irradiation with light of a suitable wavelength. As the molecules return from the excited state to their basic state, they release the absorbed energy again in the form of fluorescence, whereupon the latter is quenched by interaction with the analyte. For the detection of the fluorescence-quenching effect, it is important that the fluorescence has sufficient intensity or energy which is commensurate with the intensity of the irradiation.
In principle, it is possible to determine many other analytes in a fluid measurement medium by fluorescence quenching, as long as the signal substance, for example a fluorophore, is sensitive in regard to the analyte of interest. The term “fluid” in this context encompasses liquids and gases as well as mixtures thereof.
These sensors, which contain a signal substance, have the disadvantage that independent of the field of application and, in particular independent of the measurement medium, the fluorescence of the signal substance will weaken over time, and thus the intensity of the fluorescence-quenching effect will drop off. The decline of the fluorescence can for example be caused by the radiation being used for the measurement as well as by the cleaning methods that are employed, such as for example autoclaving or the conventional CIP- and/or SIP processes (cleaning in place, sterilizing in place). As a means to compensate for the decline in fluorescence and the associated deterioration of the measurement results, optochemical sensors operating according to this method often have exchangeable sensitive elements. While an exchange of the sensitive element restores the functionality of the sensor, it can disrupt the production process, in particular by causing additional work and causing interruptions in the reaction or process. Interruptions for the exchange of the sensor or the sensitive element may in some cases involve substantial costs and extensive efforts, especially if the sensor is installed at a less accessible location of the process system.
The aging of the sensitive element not only dictates the time for the exchange, but with advancing time in operation also comes a deterioration of the measurement accuracy of the sensitive element, which can lead to errors in the measurement results. Furthermore, in particular sensors with older sensitive elements will need to be calibrated by the user more often in order to ensure the quality and reproducibility of the measurement results.
Several known methods exist, whereby the aging of the sensitive element can be slowed down. For example, the intensity of the irradiation can be reduced, but this will cause an increase in the noise content of the raw signal, as the intensity of the fluorescence-quenching effect is being reduced at the same time. A stronger or increased noise level has the consequence that in particular lower concentrations of an analyte can no longer be determined with sufficient accuracy or cannot be determined at all. The noise could be reduced simply by taking an average over several raw or processed signals or processed signals, but this would affect, i.e. slow down, the response of the sensor. In other words, the sensor would take a longer time to react to a change of the measurement medium.
The term “raw signal” refers to the signal in its original condition as delivered by the detector. The raw signal, which is a function of the analyte content, can subsequently be converted into a processed signal which is either released directly in the form of a measurement value or can be converted further into a measurement value.
As a further possibility, the sampling rate of the measurement can be slowed down, so that the individual raw signals are separated by longer time intervals. This solution likewise slows down the response of the sensor, as fast changes or fluctuations of the analyte content in the measurement medium can hardly be detected anymore, especially if a change of the measurement medium occurs within the time between two measurement updates. Also, the sampling period can be varied only within a limited range from about 1 to 30 seconds, as the response time of the sensor would otherwise be affected too much.
It is further possible to compensate for the aging effect through arithmetic measures in the processing of the raw signals. However, the aging, more precisely the degree of aging, is specific to every application in which the sensor is being used, as the conditions to which the sensor is exposed, for example the measurement medium, analyte content, temperature, pressure, cleaning methods and/or frequency of cleaning, depend on the application. Consequently, an individual correction- or compensation function would have to be determined, implemented in a software program, and executed for each application, but with the multitude of possible applications where such sensors are used, this concept is virtually impossible to realize.
For example in US 2010/063762 A1, a method of establishing the operating life or a calibration period of a sensor is disclosed which is based on the determination and evaluation of forecast values or forecast intervals that are related to the actual service conditions of the sensor.
In US 2010/0032583 A1, a system is disclosed in which the physical location where the incident radiation meets the sensitive element can be varied and, in addition, the intensity of the incident radiation can be adjusted. This allows the magnitude or intensity of the fluorescence response to be kept at a constant level. On the one hand, as the sensitivity declines, the intensity of the incident radiation is simply raised or adjusted. On the other hand, the sensitive element can be used over a longer time period, as the position of the incident radiation relative to the sensitive element can be changed, if a currently used location on the sensitive element has been worn out to a degree where the measurement result is affected. This concept has the drawback that additional means are required in the sensor for the positioning of the incident radiation. This increases the complexity of the sensor and also makes the sensor less robust, because arrangements for shifting or adjusting an optical light path are very susceptible to mechanical stress.
Therefore, the task presents itself to develop a method for the operation of an optochemical sensor which allows the measurement performance of the sensor to be improved, wherein in addition the sensor is to maintain an unchanging level of measurement accuracy without a significant change in response time essentially independent of the aging of its sensitive element. In addition, the method should allow the sensitive element to operate with the highest possible efficiency at the lowest possible radiation exposure level.